To prepare the b╚chamel sauce, melt 2 tablespoons of the butter in a small saucepan over medium heat. Add the flour and stir for 1 minute, until bubbly. Do not allow to brown. Add the milk 1/4 cup at a time, whisking to combine each addition with the butter mixture. Increase the heat to medium-high and cook, whisking constantly, until the sauce thickens, about 2 minutes. Season with the salt, white pepper, and nutmeg and set aside, covered, while you prepare the sandwiches.
Preheat the broiler.
Place 4 of the bread slices on a work surface. Spread one side of each slice with 1/2 teaspoon of the Dijon mustard. Divide the ham and Gruy╦re slices evenly among the 4 bread slices. Top each sandwich with the remaining 4 slices of bread. Lightly butter the outsides of each sandwich with the remaining tablespoon of butter.
Heat a large skillet over medium-low heat. When hot, add the buttered sandwiches to the skillet and cook until they are golden brown on both sides, about 2 minutes per side.
Using a metal spatula, transfer the sandwiches to a small baking sheet. Divide the b╚chamel sauce evenly among the tops of the sandwiches and, using the back of a spoon, spread the sauce so that it covers most of the sandwich tops. Then sprinkle the grated Gruy╦re evenly over the sauce. Using oven mitts or pot holders, transfer the baking sheet to the oven and broil the sandwiches until the sauce is bubbly and the cheese is lightly browned in spots, about 2 to 3 minutes.
Using oven mitts or pot holders, remove the sandwiches from the oven and garnish with chives, if desired. Serve immediately.
Yield: 4 sandwiches